Here's some caliper piston size calculations. Thanks to
@superkarl for measuring his TTRS calipers.
Stock S3 ATE's
54mm
2290.22mm2
18Z Cayenne/Touareg/Q7
30/34/38mm
706.86mm2
907.92mm2
1134.11mm2
Total 2748.89mm2
18.2044% larger than stock
TTRS
40/44mm
1256.64mm2
1520.53mm2
Total 2777.17mm2
1.0235% larger than 18Z
19.219% larger than stock
17Z Cayenne/Touareg/Q7
34/36/38mm
907.92
1017.88
1134.11
Total 3059.91mm2
9.6877% larger than TTRS
10.7086% larger than 18Z
28.7728% larger than stock
I'm not sure if my calculations were correct here. I've only accounted for 3 pistons for the 18z and 17z and 2 for the TTRS/RS3. I've recalculated with all the pistons and added in RS6 calipers...
Stock S3 ATE - 1 pot
54mm x 1 = 2290.22mm
18Z Cayenne/Touareg/Q7 - 6 pots
30/34/38mm
706.86mm² * 2 = 1413.726mm²
907.92mm² * 2 = 1815.84mm²
1134.11mm² * 2 = 2268.22mm²
Total 5497.78mm²
140.0547% larger than stock
TTRS/RS3 - 4 pots
40/44mm
1256.64mm² * 2 = 2513.28mm²
1520.53mm² * 2 = 3041.06mm²
Total 5554.34mm²
1.028779% larger than 18Z
142.5243% larger than stock
C5 RS6 - 8 pots
28/32mm
615.75mm² * 4 = 2463mm²
804.25mm² * 4 = 3217mm²
5680mm²
2.262375% larger than TTRS/RS3
3.31443% larger than 18Z
148.011% larger than stock
17Z Cayenne/Touareg/Q7 - 6 pots
34/36/38mm
907.92mm² * 2 = 1815.84mm²
1017.88mm² * 2 = 2035.76mm²
1134.11mm² * 2 = 2268.22mm²
Total 6119.82mm²
7.74331% larger than RS6
10.18087% larger than TTRS/RS3
11.31439% larger than 18Z
167.2154% larger than stock
Any thoughts
@Prawn on the maths here? What are the size of the pistons on your new brakes?